The present invention relates generally to digital-to-analog converters (DAC) used in wireless communication devices and deals more particularly with a method and compensation circuit for canceling transmit DAC gain variations in such wireless communication devices due to variations in the manufacture process and temperature.
The use of wireless communication devices such as cellular telephones has increased tremendously over the last several years and such use is expected to continue to increase, particularly due to wireless connection to the Internet or other such global network services. One key component of these wireless communication devices is the digital-to-analog converter (DAC). The transmission architecture of wireless devices generally includes some type of digital circuitry which generates a bit stream which is converted to an analog signal by the DAC, and which signal is mixed up to the RF frequency for transmission by the RF power amplifier. The transmit power gain may vary from unit-to-unit due to numerous reasons, such as, for example, the tolerances of the analog components in the transmit chain including the transmit DAC, which is required between the digital base band and analog RF sections.
Typical cellular telephone designs attempt to locate the transmit DAC and RF components in different physical locations in the cellular telephone to avoid interference. One drawback of this design approach is that the process variations in the manufacturing phase of the components do not track one another. In a worst case scenario, the transmit DAC and RF components have a maximum variation in the same direction which results in a maximum total transmit gain deviation from the nominal transmit gain value.
Previous approaches for compensating for transmit gain variation from one unit to another has been to calibrate out the variation in production. One such method for calibrating out variations is to find, empirically or otherwise, control signal values that result in acceptable transmit power levels and then storing such control signal values in digital format in a non-volatile memory for subsequent retrieval during the transmission process to maintain the transmit power within acceptable levels.
Another drawback of typical cellular telephone designs is the requirement for a complex circuit design for the transmit DAC and the load circuitry driven by the DAC to compensate for transmit gain variation. The transmit DAC output variation must be minimized and the load circuitry must be capable of tolerating different input signal levels. Such requirements generally lead to higher power consumption in both the transmit DAC and the load circuitry.
Some success has been achieved in calibrating certain DAC topologies using laser trimming in production, however, such laser trimming cannot be applied to all DAC designs and topologies and does not cancel or compensate variations in the output signal due to temperature drift.
Accordingly, it is an object of the present invention to provide a method and compensation circuit for canceling transmit DAC gain variations due to DAC process and temperature variations
A method and compensation circuit for canceling transmit DAC gain variations in a wireless communication device due to manufacturing process and temperature variations is presented in accordance with the present invention wherein a transmit DAC produces an output current signal Iload proportional to a reference current Iref wherein the reference current Iref is inversely proportional to a load resistance Rload whereby the DAC output current Iload inversely tracks changes in the load resistance Rload to cancel gain variations.
In a further aspect of the invention, the reference current Iref is generated in an integrated circuit containing the load resistance Iref and by providing a reference voltage Vref to an input of an operational amplifier and coupling the output to a reference resistance Rref and a current source to generate the reference current Iref.
The load resistance Rload and reference resistance Rref are made of the same material.
In a yet further aspect of the invention, the transmit DAC includes a current steering cell for receiving the reference current Iref to produce the output current signal Iload in proportion to the reference current Iref.